1. Field of the Invention
The present invention generally relates to a method and apparatus for generating ammonia (NH3).
2. Description of Related Art
Currently, annual ammonia production exceeds 110 million metric tons, which is more than any other inorganic chemical. Approximately 80% of ammonia produced is used in agriculture. The modern, large scale manufacture of ammonia is accomplished through the Haber-Bosch process. Originally patented in 1910 (U.S. Pat. No. 971,501) by Fritz Haber and Robert Le Rossignol, the process was later commercialized by Carl Bosch and was first used for wide scale ammonia production by Germany in World War I. The Haber-Bosch process has remained fundamentally the same since that time.
The Haber-Bosch process reacts molecular hydrogen and nitrogen over an iron catalyst at high pressures (around 150 atm.) and extremely high temperatures (around 450° C.) to produce ammonia (NH3) with a 10-20% yield. The temperatures and pressures involved in this process require large energy expenditures. In addition, the molecular hydrogen feed-stock requires an extensive pre-processing step that utilizes fossil fuel, such as natural gas (methane) or liquefied petroleum gas (propane and butane) or petroleum naphtha, to produce the hydrogen. These fossil fuels are transformed into hydrogen via steam reformation and the water gas shift reaction, both of which occur at high temperatures and pressures.
The Haber-Bosch process also requires a delicate balance of temperature and pressure to optimize ammonia output. High temperatures increase the reaction rate, but also drive the equilibrium toward molecular hydrogen and nitrogen, and away from ammonia. Therefore, high pressures are applied to drive the equilibrium back towards ammonia in an attempt to maximize ammonia production. Thus, much of the energy expended in the manufacturing process is wasted on these competing processing variables.
Attempts have been made to use electrochemical synthesis to produce ammonia under standard conditions. The half-cell reactionN2+6e−→2N3−  (1)occurs at electrode potentials well below the potential that the half-cell reactionH++1e−→½H2  (2)occurs. Therefore, in reducing N2 in an attempt to produce NH3 in environments where hydrogen is present to act as a constituent in the ammonia, an overwhelming majority of the current goes towards the reduction of hydrogen rather than to the reduction of nitrogen. A number of attempts have been made to overcome this fundamental issue, such as using catalysts that are selective for the reduction of N2, and utilizing organic proton sources that have poor electrochemical activity (e.g., ethanol), and performing the reaction in highly basic aqueous solutions to limit the availability of hydrogen, but have had very limited success.
Therefore, an improved process that produces higher yields and requires less energy than the Haber-Bosch process is desired.